Vehicle speed control systems are utilized to cause the actual vehicle speed to achieve a target vehicle speed set by the vehicle driver. To control vehicle speed, engine torque is adjusted to a value that will cause the actual vehicle speed to achieve the target through manipulation of various parameters that influence engine torque.
A particular situation that has posed considerable challenge in vehicle speed control systems is controlling vehicle speed on a downgrade. In this situation, the engine must be controlled to absorb energy. In an alternate viewpoint, the engine brake torque, which is the torque at the crankshaft available to the vehicle driveline, must be controlled to a negative value.
One method of vehicle speed control controls engine airflow and cylinder deactivation. In this method, a throttle actuator controls engine airflow so that the actual vehicle speed reaches the desired vehicle speed. If an overspeed condition exists, i.e. the actual vehicle speed is greater than the desired vehicle speed and the throttle actuator reaches its lower physical limit, then cylinder deactivation is employed. In this method, the control first employs airflow control. When the airflow reaches its minimum physical value, then cylinder deactivation is used to further decrease the vehicle speed. In other words, airflow control is utilized until the airflow is moved to the minimum physical value (dictated by various air leakage paths), then other methods of reducing engine torque are utilized. One such method is disclosed in U.S. Pat. No. 4,862,367.
The inventors herein have recognized a disadvantage with the above approach. When the engine torque is controlled via airflow, a smooth and continuous engine torque is provided to smoothly control vehicle speed. However, when the airflow reaches its physical minimum value, there is a shock to the vehicle as the torque is further reduced via cylinder deactivation. This disadvantage can be moderated by decreasing the minimum physical airflow value through various mechanical design alternatives and minimization of leakage; however, there will always be some leakage airflow through the engine and thus the minimum airflow will never be zero. Therefore, when practicing the above method, there will always be a shock to the vehicle when it is necessary to deactivate cylinders to maintain the desired vehicle speed.
The inventors herein have recognized another disadvantage with the above approach. Reducing the minimum airflow limit to such low levels increases incidence of engine misfire, causing an increase in regulated emissions. Thus, the shock cannot be reduced to zero due to current emission regulations and problems associated with engine misfire as extremely low airflows.
Another vehicle speed control system attempts to solve this disadvantage by delaying cylinder deactivation for a predetermined time period or allowing vehicle overspeed situations. Such a system is disclosed in U.S. Pat. No. 5,646,851. The inventors herein have recognized a disadvantage with this approach. Using a delay does not eliminate the shock problem, it simply postpones the problem or reduces the frequency at which to problem occurs. Allowing vehicle overspeed is undesirable to a vehicle driver and thus reduces customer satisfaction.